Crosslinked Polymers with Tunable Coefficients of Thermal Expansion

ABSTRACT

Curatives and their resulting thermosets and other crosslinked polymers can reduce thermal expansion mismatch between an encapsulant and objects that are encapsulated. This can be accomplished by incorporating a negative CTE moiety into the thermoset resin or polymer backbone. The negative CTE moiety can be a thermal contractile unit that shrinks as a result of thermally induced conversion from a twist-boat to chair or cis/trans isomerization upon heating. Beyond CTE matching, other potential uses for these crosslinked polymers and thermosets include passive energy generation, energy absorption at high strain rates, mechanophores, actuators, and piezoelectric applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 17/344,717,filed Jun. 10, 2021, which claims the benefit of U.S. ProvisionalApplication No. 63/039,126, filed Jun. 15, 2020, both of which areincorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-NA0003525 awarded by the United States Department of Energy/NationalNuclear Security Administration. The Government has certain rights inthe invention.

FIELD OF THE INVENTION

The present invention relates to crosslinked polymers and, inparticular, to thermosets and other crosslinked polymers with tunablecoefficients of thermal expansion.

BACKGROUND OF THE INVENTION

Thermoset polymers are low weight, low cost, high performance materialswith excellent chemical, thermal, and mechanical stability. See J.-P.Pascault et al., Thermosetting Polymers, CRC Press: New York, 2002;Cross-Linked Polymers: Chemistry, Properties, and Applications, AmericanChemical Society: Washington, D.C., 1988, Vol. 367; and W. Brostow etal., Chapter 8 — Epoxies, In Handbook of Thermoset Plastics (ThirdEdition), Dodiuk, H.; Goodman, S. H., Eds. William Andrew Publishing:Boston, 2014; pp 191-252. In addition to their use in homogenouscomponents, thermoset polymers are frequently employed in combinationwith other materials, acting as adhesives, encapsulants, compositematrices, or barriers. See T. Engels, Chapter 10—Thermoset adhesives, InThermosets (Second Edition), Guo, Q., Ed. Elsevier: 2018; pp 341-368; F.Aguirre-Vargas, Chapter 11—Thermoset coatings, In Thermosets (SecondEdition), Guo, Q., Ed. Elsevier: 2018; pp 369-400; E. Aksu, Chapter14—Thermosets for pipeline corrosion protection, In Thermosets (SecondEdition), Guo, Q., Ed. Elsevier: 2018; pp 453-476; S. Agarwal and R. K.Gupta, Chapter 8—The use of thermosets in the building and constructionindustry, In Thermosets (Second Edition), Guo, Q., Ed. Elsevier: 2018;pp 279-302; I. Hamerton and J. Kratz, Chapter 9—The use of thermosets inmodern aerospace applications, In Thermosets (Second Edition), Guo, Q.,Ed. Elsevier: 2018; pp 303-340; M. Biron, Chapter 6—Composites, InThermosets and Composites (Second Edition), Biron, M., Ed. WilliamAndrew Publishing: Oxford, 2013; pp 299-473; A. Fangareggi and L.Bertucelli, Chapter 12—Thermoset insulation systems, In Thermosets(Second Edition), Guo, Q., Ed. Elsevier: 2018; pp 401-438; and K.Netting, Chapter 13—Thermosets for electric applications, In Thermosets(Second Edition), Guo, Q., Ed. Elsevier: 2018; pp 439-452. In thesecases, additional practical constraints are imposed upon the resultingcomposite materials, including the need to closely match the thermalexpansion behaviors of the various constituents to achieve optimalperformance.

Most solid materials experience positive thermal expansion upon heating,and the degree and rate at which this expansion occurs is referred to asthe coefficient of thermal expansion (CTE). Bulk polymers typicallypossess large, positive CTEs in comparison to other materials. Forexample, a representative CTE value of a cured epoxy is ˜55 ppm/° C.,whereas common inorganic fillers such as silica or alumina possess CTEvalues of ˜6 ppm/° C. and ˜8 ppm/° C., respectively. See H. Chun et al.,Polymer 135, 241 (2018); H. Tada et al., J. Appl. Phys. 87(9), 4189(2000); and B. Yates et al., J. Phys. C 5(10), 1046 (1972). Incomposites or devices, large differences in CTE between materials leadsto CTE mismatch, causing internal thermomechanical stresses thatultimately reduce reliability, the service life of the component and, insome cases, result in catastrophic device failure. See J. H. Okura etal., Microelectron. Reliab. 40, 1173 (2000); and J. de Vreugd et al.,Microelectron. Reliab. 50, 910 (2010). Fluoroelastomers and rubbers usedin high temperature applications, such as seals for geothermal, oil, andgas applications, can also suffer from CTE issues. As such, fine-tuningof polymer CTE represents a significant scientific challenge of interestto a variety of industries.

One strategy to address CTE mismatch is to incorporate negative thermalexpansion (NTE) materials as fillers within the polymer matrix. See W.Miller et al., J. Mater. Sci. 44(20), 5441 (2009); and K. Takenaka,Front. Chem. 6, 267 (2018). Such fillers act to depress the overall CTEof the composite. Inorganic compounds such as ZrW₂O₈ (CTE ˜−9 ppm/° C.),or GaNMn₃ (CTEs as low as −70 ppm/° C.), have been explored for thispurpose, allowing for the CTE of their respective composites to bemodulated over an order of magnitude depending on filler loading. See L.A. Neely et al., J. Mater. Sci. 49(1), 392 (2014); P. Badrinarayanan etal., Macromol. Mater. Eng. 298(2), 136 (2013); H. Wu et al., ACS Appl.Mater. Interfaces 5(19), 9478 (2013); L. M. Sullivan and C. M. Lukehart,Chem. Mater. 17(8), 2136 (2005); and J. Lin et al., Compos. Sci.Technol. 146, 177 (2017). However, despite their promise, such compositematerials are typically limited in their useful CTE window tosub-ambient temperatures. Moreover, high loadings of inorganic fillersare often required (80-90 wt %) to significantly reduce CTE values,which can hinder material processing and dramatically alter morphologyand mechanical performance.

Therefore, there is a need for thermoset and other crosslinked polymersystems that eliminate the need for fillers while achieving the CTE of afilled thermoset. In particular, there is a need for filler-lessthermosets that achieve CTE tunability to near zero ppm/° C.

SUMMARY OF THE INVENTION

The present invention is directed to thermoset and other crosslinkedsystems to reduce thermal expansion mismatch between an encapsulant andobjects that are encapsulated. In one aspect, the invention is directedto curatives comprising thermally contractile units that undergo areversible twist-boat to chair isomerization upon heating accompanied bya change in molecular volume. For example, the curative can comprise adisubstituted-dibenzocyclooctane, disubstituted-dibenzocycloheptane,disubstituted-stilbene, or disubstituted-azobenzene. For example, thedisubstitution can comprise a diamine, dicarboxylic acid, dialcohol,diisocyanate, dianhydride, diazido, or diepoxide substitution. Exemplarycuratives include diamino-dibenzocyclooctane,diazido-dibenzocyclooctane, diepoxide-dibenzocyclooctane,diazido-dibenzocyclooctane, dihydroxy-dibenzocyclooctane,diisocyanate-dibenzocyclooctane, dicarboxylicacid-substituted-dibenzocyclooctane, dianhydride-dibenzocyclooctane,diamino-dibenzocycloheptane, diamino-stilbene, and diamino-azobenzene.

As an example of the invention, diamino curatives based on thethermally-isomerizable dibenzocyclooctane (DBCO) motif were synthesized.Density-functional theory (DFT) calculations on model compounds revealeda temperature-dependent isomerization equilibrium that was mostsignificant for the cis-diamino-DBCO (DADBCO) regioisomer. These novelcuratives were used to prepare epoxy/amine thermosets. Consistent withthe DFT data, an epoxy cured with cis-DADBCO possessed a low CTE valueof 20 ppm/° C. below its T_(g) and contracted massively above T_(g),amounting to a net contraction of the material over the testedtemperature range that was highly reversible. These and relatedcompounds can be readily incorporated into a wide range of materials tofine tune CTE values.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will refer to the following drawings, whereinlike elements are referred to by like numbers.

FIG. 1 is a schematic illustration of thermally controlled isomerizationof DBCO between twist-boat and chair conformers and how theincorporation of such contractile units into thermoset materialsinfluences their thermal expansion and contraction behavior.

FIG. 2A shows a synthetic route to prepare DADBCO. FIG. 2B shows thecomponents utilized in the various epoxy formulations.

FIG. 3 shows the regioisomer structures used in DFT calculations andtheir abbreviations.

FIG. 4 is a schematic illustration of the molecular volume decreaseduring twist-boat to chair isomerization.

FIG. 5 is a graph of the relative free energies of the twist-boat andchair isomers at room temperature and 200° C. for the trans- andcis-DADBCO isomers compared to the averaged values from all six isomers.

FIG. 6A is a graph of the thermal expansion/contraction ratio of epoxysamples as a function of temperature (25-180° C. at a heating rate of10° C./min, second heating cycle). FIG. 6B is a graph of the time spentat 160° C. for the DADBCO- or EDA-cured samples.

FIGS. 7A and 7B are graphs of calculated CTE and deformation values fromdata in FIG. 6A.

FIG. 8 is a graph showing the reversibility of thermal expansion andcontraction for epoxy cured with cis-DADBCO over five heating andcooling cycles. The samples were cycled a rate of 10° C./min.

FIG. 9A shows a synthetic route to prepare adiepoxide-dibenzocyclooctane and a diazido-dibenzocyclooctane. FIG. 9Bshows a synthetic route to prepare a dicarboxylic-acid-substituteddibenzocyclooctane and a dianhydride-dibenzocyclooctane. FIG. 9C showsan ortho, ortho-cis-diamino version of tetramethylateddibenzocyclooctane.

FIG. 10A shows a synthetic route to prepare adiamino-dibenzocycloheptane. FIG. 10B shows a synthetic route to preparea diamino-stilbene. FIG. 10C shows a synthetic route to prepare adiamino-azobenzene.

FIG. 11 shows disubstituted thermally contractile units that can undergotwist-boat to chair isomerization.

FIG. 12A shows the reaction of a diamino-dibenzocyclooctane curativewith an epoxy resin to form an epoxy thermoset. FIG. 12B shows thereaction of dihydroxy-dibenzocyclooctane with an isocyanate to form apolyurethane thermoset. FIG. 12C shows the reaction of adiazido-dibenzocyclooctane curative with the perfluoroelastomer FFKM toform a crosslinked fluoroelastomer.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1 , a strategy to manipulate the CTE ofthermosets involves covalent incorporation of thermally contractileunits within a polymer network. See X. Shen et al., Nat. Chem. 5(12),1035 (2013). These “shrinking” linkages oppose thermal expansion duringheating, with the net effect of reducing the CTE of the material to nearor less than zero in some cases. As shown in the figure, one suchcontractile group is DBCO, comprising a flexible cyclooctane ringconnecting two rigid phenyl groups which undergoes a reversibletwist-boat to chair isomerization upon heating accompanied by a decreasein molecular volume. See X. Shen et al., Nat. Chem. 5(12), 1035 (2013);Z. Wang et al., Macromolecules 51(4), 1377 (2018); and W. Fu et al., J.Am. Chem. Soc. 142(39), 16651 (2020). To date, DBCO has beenincorporated into polyarylamide thermoplastics and thermosets but hasyet to be explored in other polymer systems.

An exemplary embodiment of the present invention uses the DBCO moiety toprepare di-aniline crosslinkers for use in epoxy/amine thermosets.Accordingly, the influence of the amine substitution pattern in thephenyl groups was investigated by both DFT calculations andthermomechanical analysis (TMA). Depending on the regioisomer (orpositional isomer) of DADBCO utilized as a crosslinker, epoxy thermosetscan be prepared with near-zero, or in some cases negative, CTE valuesthat feature highly reversible thermal expansion and contractionbehavior. The resulting thermoset polymers can be important in thepreparation of composite materials, especially in applications in whichdimensional precision or minimization of thermal stresses is required.

Epoxy thermosets are typically prepared from low molecular weight,epoxide-functionalized resins and di-functional crosslinkers such asdianhydrides or diamines. Epoxy/amine formulations are attractive asthey can be cured at relatively low temperatures (e.g., ≤150° C.)without additional catalysts or initiators. To prepare a curative basedon DBCO, a three-step synthetic approach was adopted, as shown in FIG.2A. In the first step, a,a′-dibromo-o-xylene was dimerized in thepresence of elemental Li to form a DBCO scaffold. See G. Franck et al.,Org. Syn. 89, 55 (2012). Soft nitration of DBCO with excess HNO₃ inCH₂Cl₂ provides the dinitro derivative (DNDBCO). See A. G. Giumanini etal., Ind. Eng. Chem. Res. 41(8), 1929 (2002). All six of the possibleregioisomers were obtained during the nitration step in approximatelyequal abundance (the ratio of ortho/meta nitro substituents was 2:3). Inthe final step, DNDBCO was hydrogenated under standard conditions toyield a mixture of DADBCO regioisomers, as shown in FIG. 3 . The trans-and cis-DADBCO regioisomers can be isolated from the DADBCO isomermixture via exhaustive chromatography.

The thermal isomerization of DBCO between the twist-boat and chairisomers has been thoroughly investigated using DFT calculations, X-raycrystallography, variable temperature NMR spectroscopy, and differentialscanning calorimetry (DSC). See I. Alkorta and J. Elguero, Struct. Chem.21(4), 885 (2010); A. Hamza, Struct. Chem. 21(4), 787 (2010); P. Domianoet al., J. Chem. Soc., Perkin Trans. 9, 1609 (1992); M. Luisa Jimeno etal., New J. Chem. 22(10), 1079 (1988); and W. Fu et al., J. Am. Chem.Soc. 142(39), 16651 (2020). However, previous studies have yet toconsider the influence of regiochemistry on the isomerizationequilibrium. Therefore, of interest is the influence that the aminosubstitution pattern has on the thermodynamics of the twist-boat tochair transition, as shown for the m,m-cis regioisomer in FIG. 4 .Toward this end, DFT calculations were carried out usingN,N,N′,N′-tetramethylated DADBCO models for each of the six possibleregioisomers shown in FIG. 3 . Calculated thermodynamic parameters andmolar volumes are provided in Table 1. DFT calculations on individualmolecules showed that the twist-boat conformation represented the globalenergy minimum at room temperature regardless of amine regiochemistry,shown in FIG. 5 . In addition, calculated molar volumes were lower forthe chair conformer relative to twist-boat in all cases. At 200° C., ΔΔG(ΔG_(200° C.)-ΔG_(25° C.)) was negative for each compound due to therelatively higher entropy of the chair conformer. However, only thecis-regioisomer possessed negative ΔG_(200° C.) for the isomerization,indicating that the equilibrium favored the chair conformer at elevatedtemperature for cis-DADBCO.

TABLE 1 DFT calculations on twist-boat and chair conformers at differenttemperatures for the various tetramethylated DADBCO isomers. Theenergies, equilibrium constants, and volumes below are given in terms ofchair relative to twist-boat (e.g., ΔH = H_(Chair) − H_(Boat)).ΔH_(25° C.) ΔG_(25° C.) ΔG_(100° C.) ΔG_(200° C.) ΔV^(a) Isomer(kcal/mol) (kcal/mol) (kcal/mol) (kcal/mol) K_(eq, 200° C.) (cm³/mol)m,m-cis 1.0 0.2 0.0 −0.2 1.2 −4.2 m,m-trans 1.1 0.7 0.6 0.5 0.6 −3.6o,m-cis 1.6 1.3 1.3 1.2 0.3 −4.9 o.m-trans 1.4 1.1 1.1 1.0 0.3 −3.0o,o-cis 0.8 0.4 0.4 0.3 0.7 −4.3 o,o-trans 1.1 1.1 1.1 1.0 0.3 −1.7average 1.2 0.8 0.7 0.5 0.6 −3.6 ^(a)Molar volumes calculated fromConnolly solvent-excluded molecular volumes using a 1.4 Å probe.

A wide variety of epoxy resins containing an epoxide functional groupcan be reacted with the amine curative, including commercially availablenovolacs, EPONEX 1510, Araldite, DER 732, EPON 826, and EPON 828. As anexample, epoxy/amine thermosets were prepared from the DADBCO isomermixture or the isolated trans- or cis-isomers and the commercial epoxyresin EPON 828 according to the formulations shown in Table 2 (chemicalstructures for the components are shown in FIG. 2B). In brief, epoxy andamine curative components were mixed at 1:1 stoichiometry (32 phr) andcured at 150° C. for 1 h. Ethylene dianiline (EDA) was employed in aseparate formulation to serve as a control. Unlike DADBCO, EDA was notexpected to undergo thermal isomerization. The curing reactions weremonitored by DSC and FT-IR spectroscopy. In both cases, curing wasmoderately faster for the trans- and cis-DADBCO isomers than for theDADBCO mixture or EDA. However, curing appeared to be complete in <30min in all cases. Thermal characterization of the as-prepared thermosetsamples was carried out using thermogravimetric analysis (TGA) and DSC.These thermal properties are summarized in Table 3. The epoxy samplesexhibited good thermal stability up to 350° C. under N₂ atmosphere andglass transition temperatures (T_(g)) that ranged from approximately100-160° C. depending on the identity of the curative and the curingmethod (i.e., cured during fast temperature ramp or isothermal curing at150° C.). Of note, the T_(g) of the epoxy cured with cis-DADBCO was10-30° C. lower than those cured with trans-DADBCO, the DADBCO mixture,or EDA. It could be the case that the additional mobility afforded bythe twist-boat to chair isomerization depressed the T_(g) in thecis-DADBCO sample.

TABLE 2 Epoxy formulations. Stoichiometry Amine Epoxy Curative EEW^(a)AEW^(b) (epoxide/amine) wt % phr^(c) Epon 828 EDA 188 53 1:1 22 28 Epon828 DADBCO 188 60 1:1 24 32 (mixed) Epon 828 trans- 188 60 1:1 24 32DADBCO Epon 828 cis- 188 60 1:1 24 32 DADBCO ^(a)Epoxide equivalentweight = ratio of MW to the number of epoxide functional groups.^(b)Amine equivalent weight = ratio of MW to the number of aminefunctional groups. ^(c)Parts per hundred resin

TABLE 3 Summary of thermal properties of cured epoxies. Cure time WeightΔH_(cure) T_(g, ramp) @ 150° C. T_(g, iso) T_(d) loss ID (J/g)^(a) (°C.)^(b) (min)^(c) (° C.)^(d) (° C.)^(e) (%)^(f) EF_(control) 453 154 26163 366 84.1 EF_(mixed) 269 134 21 142 351 80.7 EF_(trans) 112 130 15142 355 83.8 EF_(cis) 114 105 10 131 357 83.3 ^(a)Curing enthalpycalculated from the area under the curve of the temperature ramp cureDSC trace. ^(b)Calculated via DSC from the first heating cycle of theun-cured sample. ^(c)The curing time represents the time at which thederivative of the heat flow vs time curve reached zero in the DSCisothermal curing experiment. ^(d)Calculated via DSC from the secondheating cycle following isothermal curing at 150° C. for 1 h.^(e)Decomposition temperature represents the onset of the mass lossoccurring at ~200° C. for all samples. ^(f)Change in sample massfollowing water loss (measurement range ~150-600° C.).

TMA was employed to study the thermal expansion and contraction behaviorof the various epoxy samples. Sample length was monitored as a functionof either temperature (40-180° C.) or time (at 160° C.). The thermalexpansion/contraction ratios as a function of either temperature or timefor the various samples are shown in FIGS. 6A and 6B, respectively. Asshown in FIG. 6A, a linear increase in sample length is observed for theEDA-cured sample up to 155° C., after which the slope increased. Thistemperature corresponds to T_(g) and the associated softening of thematerial. Similar behavior is observed for the sample cured with theDADBCO isomer mixture. In contrast, a rapid contraction is observed forthe trans-DADBCO cured sample above its T_(g), after which the samplecontinues to thicken. This behavior is more dramatic for the cis-DADBCOsample, which contracts massively above T_(g). These observations arealso apparent in the kinetic data in FIG. 6B, showing initial expansionand then consistent size for the sample cured with EDA, expansion thenslight contraction for those cured with the DADBCO mixture ortrans-DADBCO, and slight expansion then massive contraction for thecis-DADBCO sample. This contraction occurs rapidly above T_(g), afterwhich the material maintains a relatively consistent size. Thus, thetwist-boat to chair isomerization does not appear to be kineticallycontrolled but is instead governed by the twist-boat/chair equilibrium.Indeed, the magnitudes of these contractions trend linearly with thecalculated equilibrium constants at 200° C., as shown in Table 1.

The CTEs for each sample were calculated in the range of 50-100° C.(below T_(g)). As shown in FIG. 7A, the EDA, mixed DADBCO, andtrans-DADBCO samples exhibited CTE values typical for epoxy thermosets(ca. 60 ppm/° C.). In contrast, the CTE for the cis-DADBCO sample was 20ppm/° C. Such a low CTE value is rarely observed in polymer materials.In addition, this sample showed a net contraction across the completetemperature range (20-108° C.) whereas the others expanded upon heating,as shown in FIG. 7B. The temperature at which the slope changed in theplot of expansion/contraction ratio vs. temperature, T_(c), was observedfor each sample. The T_(c) decreased from 158° C. for the mixed DADBCOsample to 143° C. for the sample cured with cis-DADBCO. A summary of thethermomechanical behavior of the cured epoxies in shown in Table 4.These data trend with the calculated equilibrium constants, shown inTable 1, suggesting a relationship to the position of thetwist-boat/chair equilibrium.

TABLE 4 Summary of thermomechanical behavior of cured epoxies. 1^(st)heating cycle 2^(nd) heating cycle CTE1^(a) CTE2^(a) T_(c) ^(b) CTE1^(a)CTE2^(a) T_(c) ^(b) Contraction^(c) Deform^(d) Curative (ppm/° C.)(ppm/° C.) (° C.) (ppm/° C.) (ppm/° C.) (° C.) (%) (%) EDA 71 ± 2 130 ±20  162 ± 4 65 ± 1 130 ± 10 158 ± 3 N/A 11.8 ± 0.4 DADBCO 58 ± 4 110 ±30  157 ± 3 62 ± 3 140 ± 20 158 ± 3 −0.3 13.6 ± 4.1 mixture trans- 64 ±3 160 ± 120  134 ± 13 59 ± 3 170 ± 60 149 ± 5 −2.1 13.6 ± 2.3 DADBCOcis- −2 ± 5 340 ± 140 139 ± 4 16 ± 5 −250 ± 80  143 ± 1 −9.3 −5.6 ± 3.1DADBCO ^(a)CTE values calculated from the 1^(st) (50-100° C.) and 2^(nd)(150-170° C.) slopes of the various TMA heating curves. ^(b)Temperatureat which the slope of the TMA curve changes between the reported CTEvalues. ^(c)The maximum contraction of the sample experiencedimmediately after T_(c) in the isothermal experiments. ^(d)The totalchange in the sample dimension across the complete temperature range(20-180° C.).

The reversibility of the thermal contraction behavior was examined. Thecis-DADBCO sample was subjected to five heating and cooling cycles from20-180° C. at 10° C./min. As shown in FIG. 8 , sample contraction wasobserved during the heating ramp of each cycle followed by re-expansionduring cooling, demonstrating good reversibility. The magnitude of thelength changes associated with expansion/contraction generally decreasedwith each cycle, perhaps due to thermal re-arrangement and/or relaxationof the epoxy resin constituents. The large initial contraction uponheating could be recovered via physical aging for at least 24 h at roomtemperature.

Taken together, data from DFT calculations and TMA experiments show thatthe macroscopic thermal expansion/contraction behavior of the curedepoxy samples depends on the equilibrium of isomerization of the DBCOmoieties on the molecular level. In particular, a large shift frompositive to negative ΔG with temperature for cis-DADBCO corresponded toreduced CTE below T_(g) and a large contraction above T_(g) in the curedepoxy.

The synthetic approach to reducing CTE can be applied to a variety ofthermosetting resins, including but not limited to epoxy resins,acrylates, methacrylates, unsaturated polyesters, vinyl esters, andurethanes, and other crosslinked polymer systems to provide a tunablecoefficient of thermal expansion. The curative can comprise any moleculethat can undergo a twist boat/chair or cis/trans isomerization which isenergetically favored to flip to a secondary conformation at elevatedtemperatures in which the energetically favorable conformation has asmaller volume than the conformation at room temperature. The curativecan be substituted with a variety of reactive groups, including amines,carboxylic acids, alcohols, isocyanates, anhydrides, epoxides, etc.

Crosslinking of an epoxy resin with diamino-dibenzocyclooctane to forman epoxy thermoset is shown in FIG. 12A. The approach can be used withother epoxy curatives that can crosslink with epoxy resins to formthermosets. FIG. 9A shows a synthetic route to prepare diepoxide- anddiazido-dibenzocyclooctanes. In this scheme, diamino-dibenzocyclooctaneis reacted with sodium nitrite in hydrochloric acid to form a diazocompound in situ. Addition of sodium azide to the diazo compound yieldsthe diazido-dibenzocyclooctane. Addition of water to the diazo compoundyields a diphenol. O-Alkylation of the diphenol with epichlorohydrinyields the diglycidyl ether-dibenzocyclooctane.

FIG. 9B shows a synthetic route to prepare dicarboxylic-acid-substituteddibenzocyclooctane and dianhydride-dibenzocyclooctane. Formylation ofdibenzocyclooctadiene using dichloromethyl methyl ether and tin (IV)chloride yields a dialdehyde. Many regioisomers of the dialdehyde arepossible, but they can be separated with column chromatography.Oxidation of the dialdehyde with potassium permanganate yields thedicarboxcylic-acid-substituted dibenzocyclooctane. Thedicarboxcylic-acid-substituted dibenzocyclooctane can be acylated withchloroacetone to give the dianhydride-dibenzocyclooctane.

The phenyl rings of a disubstituted dibenzocyclooctane can be furthersubstituted with one or more alkyl groups such that the molecule canstill undergo reversible twist-boat to chair isomerization. FIG. 9Cshows an exemplary ortho-ortho-cis diamino version of tetramethylateddibenzocyclooctane.

In addition, the approach can be also used with other thermallycontractile units that undergo a reversible twist-boat to chairisomerization upon heating accompanied by a change in molecular volume.FIG. 10A shows a synthetic route to prepare a diamino-substituteddibenzocycloheptane. Dibenzosuberone can be nitrated using a mixture ofnitric and sulfuric acid to give dinitro-dibenzosuberone as the majorregioisomer. The nitro groups on dinitro-dibenzosuberone can be reducedwith tin (II) chloride in a hydrochloric acid and acetic acid solvent toyield diamino-dibenzosuberone. The ketone in diamino-dibenzosuberone canbe completely reduced to the methylene group using a combination oflithium aluminum hydride and aluminum trichloride in an ethereal solventto give diamino-dibenzocycloheptane.

Other thermally contractile units that can undergo cis/transisomerization upon heating include stilbene and azobenzene. FIG. 10Bshows a synthetic route to prepare diamino-stilbene. 4-vinylaniline canbe reacted with di-t-butyl dicarbonate (Boc₂O) to give a protectedamine. Metastasis of the protected amine using a second-generationHoveyda-Grubbs (HG2) catalyst yields the predominately trans isomer ofthe N-protected stilbene. The stilbene can be exposed to acid to removethe t-butyloxycarbonyl (Boc) protecting group and give diamino stilbene.FIG. 10C shows a synthetic route to prepare diamino-azobenzene.4′-aminoacetanilide can be reacted with boric acid and sodium perboratein acetic acid to yield the substituted azo compound. Addition of acidto the substituted azo compound removes the acyl groups to yield4,4′-diaminohydrazobenzene.

Still other thermally contractile units that can undergo twist-boat tochair isomerization are shown in FIG. 11 . These structures are notlimited to the exemplary isomers shown but can include other isomersthat exhibit favorable conformal changes upon heating. Further, thetertiary amines shown in these examples can be replaced with any of theother functional groups described above, including carboxylic acids,alcohols, isocyanates, epoxides, anhidrides, etc., to providebisubstituted structures with different functionalities.

The synthetic approaches also lend themselves to the creation ofDBCO-dialcohol or DBCO-diisocyanate curatives which can reacted into thebackbones of polyurethanes. For example, an isocyanate can becrosslinked with dihydroxy-dibenzocyclooctane, as shown in FIG. 12B, ora polyol can be crosslinked with diisocyanate-dibenzocyclooctane toprovide a polyurethane thermoset.

The approach also can be used to synthesize other crosslinked polymers,such as crosslinked rubbers and fluoroelastomers. For example, theperfluoroelastomer FFKM containing some amount of crosslinkable monomer,such as a cyano-functionalized co-monomer, can be crosslinked withdiazido-dibenzocyclooctane to form a crosslinked fluoroelastomer, asshown in FIG. 12C. See G. Tillet et al., J. Polym. Sci. A Polym. Chem.53, 1171 (2015).

The present invention has been described as crosslinked polymers withtunable coefficients of thermal expansion. It will be understood thatthe above description is merely illustrative of the applications of theprinciples of the present invention, the scope of which is to bedetermined by the claims viewed in light of the specification. Othervariants and modifications of the invention will be apparent to those ofskill in the art.

We claim:
 1. An epoxy thermoset comprising an epoxy resin crosslinkedwith a disubstituted-dibenzocyclooctane,disubstituted-dibenzocycloheptane, disubstituted-stilbene, ordisubstituted-azobenzene curative.
 2. The epoxy thermoset of claim 1,wherein the disubstituted-dibenzocyclooctane curative comprises adiamino-dibenzocyclooctane, diepoxide-dibenzocyclooctane,diazido-dibenzocyclooctane, dicarboxylicacid-substituted-dibenzocyclooctane, or dianhydride-dibenzocyclooctanecurative.
 3. The epoxy thermoset of claim 2, wherein thediamino-dibenzocyclooctane comprises cis-diamino-dibenzocyclooctane.